In recent years a system to improve the accuracy of pseudo range-determined position identifications by the global position system to enable such higher accuracy applications as aircraft landings and harbor navigations has been implemented. This supplement to the global position system is identified as the Wide Area Differential GPS (DGPS) or more commonly as the Wide Area Augmentation System (WAAS). In principle this Wide Area Augmentation System involves an additional station, possibly a satellite station, in which there is computed a range error correction for each global position system satellite that is in view from a particular earth location. After computing these range error corrections the correction data is broadcast to GPS users located in the satellite use area. One class of input data employed by a Wide Area Augmentation System station in computing range error corrections is data received from a plurality of ground stations also located in the satellite use area.
The pseudo range determination, with its use of the Coarse Acquisition Code (C/A Code) A portion of the information available from a GPS signal, is of course only one of a plurality of position determinations that become available with the aid of GPS signals. The P Code portion of a GPS signal provides another position determination arrangement, an arrangement that is dependent on a security coding arrangement provided-for in the GPS signal format. The P code portion of the GPS signal has been reserved for military and other government uses. Generally this P code portion of the GPS signal may be located in time within the GPS signal with the aid of the C/A code and following such location the C/A code becomes of little additional value in the processing. A system having some similarity to but different protocols from the GPS system has been deployed by former Soviet Union nations. The present invention is believed to also be usable in conjunction with such system.
As is more fully disclosed in the paragraphs following herein we have found that data received from an implemented Wide Area Augmentation System station is also useful for correcting another error often encountered in a C/A code based global position system receiver, i.e., the frequency of the receiver""s sampling clock. Since this receiver sampling clock frequency determines the accuracy of one position location factor, the pseudo-range determination, made by the receiver, i.e., the determination of distance between the receiver and the global position system satellite, an error in this range determination degrades a user position computation made by global position system techniques and is therefore desirably avoided.
The pseudo-range when calculated by a GPS receiver depends on the accuracy of the receiver sampling frequency, which in turn depends on the accuracy of a receiver oscillator. If the sampling frequency is inaccurate, the pseudo-range will have a directly related error, which of course degrades the calculated user position. An accurate oscillator is however expensive to achieve in a GPS receiver. The accuracy of a low cost oscillator is usually less than desired in that the output frequency can be off from the specified value. It is thus important to know the true sampling frequency of a receiver in order to correct the accomplished calculations of user position. One way to obtain this sampling frequency knowledge is to use a frequency counter as may be accomplished in the laboratory; such counter equipment is however unavailable to many potential users of a GPS receiver.
The Wide Area Augmentation System (WAAS) provides an alternative arrangement for accurately determining receiver sampling frequency; this arrangement is the essence of the present invention. The Wide Area Augmentation System uses geo-stationary satellites, satellites receiving measured data from a plurality of ground stations, and subsequently transmits pseudo-range correction information to adjacent GPS users. Since Wide Area Augmentation System satellites are disposed in geo-stationary earth orbits, the Doppler frequency shift caused by satellite motion is quite small in such a signal i.e., on the order of a few hundred Hertz at the greatest; the Doppler frequency shift of an earth bound Wide Area Augmentation System station is of course zero. The signal transmitted by a WAAS station, i.e., either a satellite or earth bound station, can thus be used to calibrate the sampling frequency of a GPS receiver in the manner disclosed below.
With respect to accuracy of a Wide Area Augmentation System-based determination of receiver sampling frequency, the WAAS signal frequency is 1575.42 megahertz and the sampling frequency of a GPS receiver is in the neighborhood of 5 megahertz. The ratio of these two frequencies is about 300; thus, a 100-Hertz inaccuracy in the WAAS (as may for example result from the quite small Doppler effects) will reflect as about 0.3 Hertz (100/300) of frequency inaccuracy with respect to the crystal of a global position system receiver. The error of the sampling frequency measured by the approach of the present invention should therefore be less than 1 Hertz. It is significant however that the WAAS signal can be rather weak for users in certain areas. The noise component in the received WAAS signal can therefore be a significant factor in determining an actual global position system receiver sampling frequency and is considered in our invention as is disclosed below herein.
The present invention provides a calibration arrangement for the sampling frequency clock of a global position system receiver, a calibration arrangement based on the accurate atomic clock frequency determination and the low Doppler component reference available in a Wide Area Augmentation System signal.
It is another object of the invention to provide a new use for a Wide Area Augmentation System station.
It is therefore an object of the invention to provide a new use for existing information in a Wide Area Augmentation System signal.
It is another object of the invention to provide a convenient and accurate calibration arrangement for the sampling frequency clock of a global position system receiver.
It is another object of the invention to provide a calibration arrangement for the sampling frequency clock of a global position system receiver, a calibration arrangement exclusive of or tolerant of real world effects including Doppler frequency change effects and unfavorable signal to noise ratios.
It is another object of the invention to seize upon the advantages that the geo-stationary satellite orbit and low Doppler frequency shift a Wide Area Augmentation System signal inherently make available for exploitation.
It is another object of the invention to provide a signal noise considered arrangement for comparing the frequency of two differing signals.
These and other objects of the invention will become apparent as the description of the representative embodiments proceeds.
These and other objects of the invention are achieved by the method of providing pseudo range determinations of limited receiver sampling frequency inaccuracy influence from a global position system radio receiver, said method comprising the steps of:
coupling signals received from at least one global position system satellite and signals received from a Wide Area Augmentation System station into said global position system radio receiver;
extracting signals representing radio receiver sampling frequency from a global position system satellite signal processing portion of said global position system radio receiver;
educing signals representing a course acquisition code portion of said Wide Area Augmentation System signal received from said Wide Area Augmentation System station;
comparing said signals representing radio receiver sampling frequency with a selected number of said signals representing a course acquisition code portion of said Wide Area Augmentation System signal;
correcting pseudo range computations accomplished in said satellite signal processing portion of said global position system radio receiver, computations influenced by said global position system radio receiver sampling frequency, by a correcting factor responsive to differences between said signals representing radio receiver sampling frequency and signals representing a course acquisition code portion of said Wide Area Augmentation System signal as determined in said comparing step.